1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor film that is formed on a substrate having an insulating surface and that has a crystal structure, and to a method of manufacturing a semiconductor device using this semiconductor film as an active layer. More specifically, the present invention relates to a semiconductor device using a crystalline semiconductor film as an active layer and to an electronic apparatus using the semiconductor device as a display unit.
2. Description of the Related Art
In recent years, some have sought advantage in forming a channel forming region from a single crystal, which has less defects, with regard to improvement in mobility of low temperature polycrystalline silicon and to drop in Ioff. A related technology has thus been developed which includes forming an amorphous semiconductor film on a light transmissive substrate with an insulating surface, and crystallizing the film by laser annealing, thermal annealing, etc., to use the obtained crystalline semiconductor film as an active layer of a thin film transistor (hereinafter referred to as TFT).
Laser annealing is known as a crystallizing technology capable of imparting high energy only to an amorphous semiconductor film to crystallize the film. In particular, an excimer laser emitting shortwave light of 400 nm wavelength or less is a representative laser that has been used since early stages of development of laser annealing echnology. In addition to the excimer laser annealing, a technique using YAG laser which is a solid state laser has been developed lately. In the laser annealing mentioned above, a laser beam is processed by an optical system so as to take a spot-like shape or a linear shape on an irradiation surface, and the irradiation surface on a substrate is scanned with the processed laser light (irradiation position of laser light is moved relative to the irradiation surface). For instance, excimer laser annealing using linear laser light is capable of annealing the entire irradiation surface with laser by merely scanning in one direction that is perpendicular to the longitudinal direction of the surface. The excimer laser annealing using linear laser light is thus superior in productivity and is becoming the mainstream in techniques of manufacturing liquid crystal display devices using TFTs. This laser annealing technique has realized a monolithic type liquid crystal display device in which TFTs constituting a pixel portion (pixel TFT) and TFTs constituting driver circuits provided in the periphery of the pixel portion are formed on one glass substrate.
However, the crystalline semiconductor film formed by laser annealing is an accumulation of plural crystal grains, and the crystal grains are positioned at random in the film and the size thereof is irregular. In the TFT fabricated on the glass substrate, the crystalline semiconductor film is divided and formed into an island-like pattern with the intention of partitioning elements. It is therefore impossible to form the TFT with the position and the size of crystal grains specified. The interface of the crystal grains (crystal grain boundary) has a recombination center and a trapping center caused by the amorphous structure, crystal defects, etc., which are factors in degrading the current transportation characteristic of carriers. The potential level in the crystal grain boundary also affects this characteristic.
The crystallinity of a semiconductor film in a channel forming region has a great influence on a TFT characteristic. However, it is almost impossible to form the channel forming region from a single crystal semiconductor film while removing the adverse influence of the crystal grain boundary.
Attempts at growing the crystal grains larger have been made in order to solve this problem. For instance, a method of laser annealing has been reported in “High-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Method”, K. Shimizu, O. Sugiwara and M. Matsumura, IEEE Transactions on Electron Devices, vol. 40, No. 1, p.p. 112–117, 1993. According to the method, a three-layer film consisting of Si, SiO2, and Si is formed on the substrate and both sides of the device, i.e., the three-layer film side and the substrate side, are irradiated with excimer laser light. The article states that the method is capable of enlarging the size of the crystal grains by laser light irradiation with a certain energy intensity.
The method proposed by K. Shimizu et al., is characterized in that a thermal characteristic of a base material of an amorphous silicon film is changed locally to control the heat flow to the substrate and to introduce a temperature gradient. In order to introduce the temperature gradient, a three-layer structure consisting of a high melting point metal layer, a silicon oxide layer, and a semiconductor film is formed on a glass substrate. Structurally speaking, to fabricate a top gate type TFT with this semiconductor layer as an active layer is not impossible. However, the silicon oxide film provided between the semiconductor film and the high melting point metal film generates parasitic capacitance and increases power consumption, making it difficult to obtain a TFT operating at a high speed.
Other methods such as a method that uses lasers having a phase difference and the step irradiation method, also have a problem and require a complicated laser apparatus. In addition, when applied to crystallization of driving elements of a liquid crystal panel having a driver circuit incorporated therein, the methods may not always be successful in enabling all part of the channel forming region to have a large grain, nor in crystallizing them into a single crystal, for the elements are usually arranged irregularly, not with regular intervals.
There is another method that is a combination of the dual beam method and the three-layer island structure. (The dual beam method is a method in which an amorphous semiconductor film is crystallized by irradiating each side of a substrate with a laser, or by irradiating one side of a substrate with a laser and then irradiating the other side of the substrate with the laser transmitted through the substrate and reflected by a mirror or the like.) When applied to crystallization of driving elements of a liquid crystal panel having a driver circuit incorporated therein, the combination method is capable of crystallizing a designated site into a single crystal, but is not good at growing a crystal grain to as large a grain size as 5 μm or more. Therefore the method is not suitable for manufacturing a thin film transistor whose channel width is wide. The method also generates a parasitic capacitance between a metal and Si, causing signal delay. Furthermore, the method has a problem of peeling because the temperature sometimes reaches high at a time of irradiation depending on the metal material used.
A method in which a base film is formed from a highly heat conductive insulating film has an advantage in that a parasitic capacitance is not generated between the metal and Si. However, the method requires a development of a highly heat conductive insulating film that is stable.